The present invention relates to a four-stroke cycle internal combustion engine, and particularly to a small-size four-stroke cycle internal combustion engine used for hand-held operating machines such as a portable grass trimmer.
Heretofore, there has been known a four-stroke cycle internal combustion engine in which an oil reservoir chamber for reserving lubricating oil is arranged below a crank chamber having a crankshaft therein, and oil mist formed in the oil reservoir chamber is supplied into an interior of the crank chamber through an opening formed between the oil reservoir chamber and the crank chamber and supplied into a valve chamber through an oil mist passage providing fluid communication between the oil reservoir chamber and a valve chamber.
For example, Japanese Patent Laid-Open Publication No. 2000-73729 (corresponding to U.S. Pat. No. 6,202,613) discloses a four-stroke cycle internal combustion engine comprising a piston, and a connecting rod having a small end coupled to the piston and a crankshaft coupled to a big end of the connecting rod. An oil reservoir chamber for storing lubricating oil is provided below a crank chamber housing the crankshaft and the connecting rod. The crank chamber and the oil reservoir chamber are separated by a partition wall. The big end of the connecting rod is provided with an oil dipper which extends straight downward to splash and atomize the lubricating oil. The partition wall is formed with an opening which allows the oil dipper to swingably move in conjunction with a swing motion of the connecting rod without any interference with the partition wall. Further, a cylinder block is formed with an oil mist passage extending between the oil reservoir chamber and the valve chamber. Additionally, an oil separator chamber is provided above the valve chamber to separate a residual oil after lubricating the valve chamber, and a vertical conduit is formed in the cylinder block to provide fluid communication between the valve chamber and the crank chamber.
Based on the motion of the connecting rod, the oil dipper swings and moves into and out of the oil reservoir chamber through the opening to splash the lubricating oil. This motion generates oil mist. In response to the pressure fluctuation in the crank chamber caused by a reciprocating motion of the piston, the generated oil mist is pushed out of the oil reservoir chamber into the crank chamber, and simultaneously out of the oil reservoir chamber and into the valve chamber through the oil mist passage. Each portion in the crank chamber and the valve chamber is lubricated by the oil mist, and then the residual oil mist is transferred into the oil separator chamber. In the oil separate chamber, the oil mist is condensed in a liquid form of lubricating oil, and returned into the oil reservoir chamber through the vertical conduit and the crank chamber.
Generally, in a high-speed operation of a four-stroke cycle internal combustion engine, the increased temperature of lubricating oil causes its viscosity to be lowered. Thus, in order to maintain a normal operation of the engine, it is necessary to supply an oil mist of high concentration to the crank chamber so as to sufficiently lubricate the piston, connecting rod, crankshaft and the like.
In this case, the oil mist of high concentration supplied to the crank chamber is delivered to the valve chamber through the oil mist passage. However, a cam and other parts in the valve chamber have relatively low necessity for lubricating oil because such parts are not heated as much as those in the crank chamber. The four-stroke cycle internal combustion engine disclosed in the aforementioned Publication advantageously includes the oil separator chamber to separate the oil contained in the residual oil mist of high concentration and return the separated oil to the oil reservoir chamber. However, in view of manufacturing cost and its maintenance, it is desired to provide a four-stroke cycle internal combustion engine having a sufficiently simplified structure. On the other hand, there is another four-stroke cycle internal combustion engine having a breather at the valve chamber. Residual oil mist can be discharged from the breather to a filter, and the filtered air is released to the outside air. In this case, if the oil mist contains a large quantity of oil, the filter will be saturated in a short time, and the oil will not be sufficiently trapped by the filter. This causes an undesirable release of oil mist into the outside air, and increased oil consumption. Further, it is inconveniently required to replace the filter frequently.
It is therefore an object of the present invention to provide a four-stroke cycle internal combustion engine capable of supplying an adequate amount of lubricating oil to both of a crank chamber and valve chamber thereof.
In order to achieve this object, the present invention provides a four-stroke cycle internal combustion engine, comprising: a crankshaft provided within a crank chamber; an oil reservoir arranged below the crank chamber and for storing lubricating oil; a valve chamber which is in communication with the oil reservoir through an oil mist passage, oil mist being formed in the oil reservoir and supplied to an inside of the crank chamber and to the valve chamber through the oil mist passage; the oil mist passage having a pre-separation oil mist passage section formed in the crankshaft, the pre-separation oil mist passage section being in fluid communication with the oil reservoir at one end and open in a radially outward direction of the crankshaft at the other end; a cylindrical centrifugal-separation chamber surrounding an outer-periphery of the crankshaft where the other end is formed; a post-separation oil mist passage section extending from the centrifugal-separation chamber in the opposite direction of a rotational direction of the crankshaft and in fluid communication with the valve chamber; and an oil return passage in fluid communication with the centrifugal-separation chamber and the oil reservoir, the oil return passage extending from the centrifugal-separation chamber in the same direction as the rotational direction of the crankshaft.
In the present invention, the oil mist formed in the oil reservoir chamber is transferred into the centrifugal-separation chamber through the pre-separation oil mist passage section by a positive pressure in the crank chamber during the downward movement of the piston. The oil mist is discharged from the opening of the crankshaft into the centrifugal-separation chamber, and is then directed to flow in the same direction as the rotational direction of the crankshaft by the rotational movement of the crankshaft. The oil mist flows around the crankshaft along the cylindrical inner-periphery of the centrifugal-separation chamber within the centrifugal-separation chamber. During this stage, among oil particles included in the oil mist, smaller and lighter oil particles are separated from larger and heavier oil particles due to a difference in centrifugal force acting thereon. That is, the smaller and lighter oil particles have a lower inertia force within the centrifugal-separation chamber. Thus, when a positive pressure of the crank chamber is introduced into the centrifugal-separation chamber, a push-out force of the positive pressure overrides the inertia force, and the smaller and lighter oil particles are pushed out of the centrifugal-separation chamber into the post-separation oil mist passage section. Then, the lighter oil particles are supplied to the valve chamber through the post-separation oil mist passage section.
On the other hand, the oil mist of the separated larger oil particles is relatively heavy and thereby has a higher inertia force. Thus, even if the positive pressure of the crank chamber is introduced into the centrifugal-separation chamber, the inertia force will override the push-out force of the positive pressure and most of the larger oil particles remain within the centrifugal-separation chamber without being transferred into the post-separation oil mist passage section. Thus, during the course of the movement in the centrifugal-separation chamber, the oil mist of the larger oil particles move along the inner-periphery of the centrifugal-separation chamber according to the inertia force, and then flows from the centrifugal-separation chamber into the oil return passage extending in the same direction as the rotational direction of the crankshaft, followed by returning into the oil reservoir.
According to the present invention, in the stage before the oil mist is introduced into the valve chamber, the smaller and lighter oil particles among oil particles contained in the oil mist are separated from the larger and heavier oil particles by the centrifugal-separation chamber. Thus, the crank chamber and valve chamber can be supplied with a larger quantity and smaller quantity of lubricating oil, respectively. This allows the crank chamber and valve chamber to be lubricated with an adequate amount of lubricating oil. Furthermore, the centrifugal force of the centrifugal-separation chamber is generated by using the rotational movement of the crankshaft being an existing part. This provides a simplified structure. In addition, since the oil is returned into the oil reservoir through the oil return passage, the lubricating oil can be efficiently utilized.